1. Field of the Invention
The present invention relates to an information-recording medium on which information is reproduced by transferring magnetization information recorded in a recording layer to a reproducing layer. The present invention also relates to a recording and reproducing method to be performed on the information-recording medium, and a recording and reproducing apparatus to be used for the information-recording medium. In particular, the present invention relates to an information-recording medium in which minute recording magnetic domains are easily formed. The present invention also relates to a method for recording information and a method for reproducing information by applying a recording magnetic field and a reproducing magnetic field by means of a novel magnetic field-applying method respectively. Further, the present invention relates to a recording apparatus and a reproducing apparatus to be preferably used for the above as well.
2. Description of the Related Art
The optical recording medium such as a magneto-optical recording medium is known as an external memory for the computer or the like. The magneto-optical recording medium can deal with a large capacity of data such as animation image and voice data, and hence it is frequently used as a recording medium suitable for the multimedia age. In general, the recording on the magneto-optical recording medium is performed by utilizing the temperature characteristic of the coercive force of a magnetic material which constitutes a recording layer. That is, a recording magnetic field, which has a direction opposite to a direction in the initial state, is applied to the magneto-optical recording medium in which the direction of magnetization in the recording layer is aligned in the certain direction in the initial state. Simultaneously, a recording light beam is radiated to locally heat the recording layer. Accordingly, the coercive force is lowered in a heated area of the recording layer to cause inversion into the direction of the recording magnetic field. After that, the magnetization of the recording layer is settled in a stable state while maintaining the inversion in the cooling process to form the recording magnetic domain. Thus, the information is recorded as magnetization information in the recording layer.
In recent years, it is demanded to further increase the storage capacity of the magneto-optical recording medium. Those suggested to realize such a demand include, for example, the mark edge recording system and the light pulse magnetic field modulation system. In the mark edge recording system, only one piece of information is not given to one recording mark in the recording layer, but pieces of information are given to a leading edge and a trailing edge of one recording mark, respectively. That is, the recording density in the linear direction is improved by giving two pieces of information to one recording mark. According to this method, it is possible to achieve a high density of about 1.5-fold, even in consideration of the separation limit of the reproducing light beam during reproduction.
On the other hand, the light pulse magnetic field modulation system resides in a method in which a recording magnetic field having a polarity corresponding to a recording signal is applied while radiating a pulsed recording light beam in synchronization with a recording clock. According to this method, it is possible to form a minute recording magnetic domain in a recording layer, and the recording density is improved.
When it is intended to further advance the realization of the high density of the magneto-optical recording medium, it is considered to be extremely important to control the recording magnetic field and the recording light beam, for example, in the case of the light pulse magnetic field modulation system. That is, in order to form a minute recording magnetic domain in the recording layer, it is necessary to strictly control the power of the recording light beam which determines the size of the recording magnetic domain, and the power of the recording magnetic field which is used to invert such a recording magnetic domain respectively. Therefore, the power margin (write power margin) is narrowed for the recording light beam and the recording magnetic field. For this reason, it has been strongly demanded to realize a novel recording method in which the high density recording can be performed with ease.
On the other hand, a problem also arises when it is intended to reproduce the minute recording magnetic domain formed in the recording layer. In general, the spot diameter of the reproducing light beam is restricted by the limit of NA of a lens carried on an optical head, and it cannot be decreased to be smaller than the above. For this reason, it is impossible to individually reproduce a plurality of minute magnetic domains existing within the spot of the reproducing light beam. That is, the individual minute magnetic domains cannot be reproduced, because of the shortage of resolution of the reproducing light beam. Therefore, it has been required to reproduce the minute magnetic domain by using the reproducing spot diameter having a certain size in the present circumstances.
The magnetically induced super resolution technique (MSR) has been suggested as a method for dissolving the foregoing problem, as described, for example, in Journal of Magnetic Society of Japan, Vol. 17 Supplement No. S1, pp. 201 (1993). In this technique, even when two recording magnetic domains are present within the reproducing light beam spot, one of the magnetic domains is masked to disappear so that the effective field of vision is narrowed. Thus, the other recording magnetic domain can be reproduced. The use of this technique makes it possible to improve the reproducing resolution without actually reducing the diameter of the reproducing light beam spot. However, even when the magnetically induced super resolution technique is used, the reproduced signal intensity obtained from each magnetic domain is not changed. Therefore, C/N of the reproduced signal is still low.
The present inventors have disclosed, in a patent document of International Publication No. WO98/02878, a magneto-optical recording medium comprising a magnetically magnifying reproducing layer and a recording layer on a substrate, in which a minute magnetic domain in the recording layer is individually transferred to the reproducing layer during reproduction, a reproducing magnetic field is applied, and thus the magnetic domain transferred to the reproducing layer can be magnified and reproduced. When the magneto-optical recording medium is used, the reproduced signal intensity is remarkably increased, because the magnetic domain transferred to the magnetically magnifying reproducing layer is magnified to have a size which is approximate to the light spot size. This technique is called xe2x80x9cMAMMOSxe2x80x9d (Magnetic Amplifying Magneto-Optical System), which dissolves the foregoing problem of the magnetically induced super resolution technique concerning the reproducing C/N of the minute magnetic domain.
In MSR and MAMMOS as described above, the reproducing magnetic field is applied perpendicularly to the medium, and the recording magnetic domain, which is located in the high temperature area of the recording layer heated by being irradiated with the reproducing light beam, is transferred to the reproducing layer to read the information. However, when the reproduction is performed on an information-recording medium which is subjected to the super high density recording, it is feared that any reproduced signal is detected as a result of inversion of a magnetic domain in the reproducing layer in the recording direction effected by a large reproducing magnetic field, although no recording magnetic domain exists in the high temperature area of the recording layer heated by being irradiated with the reproducing light beam. On the other hand, in order to ensure that a magnetic domain in the reproducing layer in the recording direction is inverted by a certain level of reproducing magnetic field when a recording magnetic domain exists in the high temperature area of the recording layer heated by being irradiated with the reproducing light beam. Accordingly, it is necessary to strictly control the powers of the reproducing light beam and the reproducing magnetic field within a limited ranges. Therefore, a problem arises in that the power margin (read power margin) is narrowed for the reproducing light beam and the reproducing magnetic field.
The present invention has been made in order to solve the problems involved in the conventional technique as described above. A first object of the present invention is to provide a recording method which makes it possible to record information with a wide recording light beam power margin and a wide recording magnetic field power margin.
A second object of the present invention is to provide a novel recording method which makes it possible to record information in a recording layer at a super high density.
A third object of the present invention is to provide a recording apparatus which makes it possible to form a minute recording magnetic domain in a recording layer of an information-recording medium easily and highly accurately.
A fourth object of the present invention is to provide an information-recording medium which makes it possible to reproduce information with a wide reproducing light beam power margin and a wide reproducing magnetic field power margin.
A fifth object of the present invention is to provide a reproducing method which makes it possible to reproduce a recording magnetic domain recorded at a high density in a recording layer reliably with an amplified reproduced signal intensity.
A sixth object of the present invention is to provide a reproducing apparatus which makes it possible to individually and reliably reproduce a minute magnetic domain formed on a medium even when the medium is an information-recording medium subjected to high density recording.
According to a firs aspect of the present invention, there is provided a recording method on an information-recording medium having a plurality of tracks for recording information by applying a recording magnetic field by using a magnetic field-generating source to an area on a surface of the information-recording medium irradiated with a recording light beam while radiating the recording light beam onto the surface of the disk-shaped information-recording medium including a recording layer, the method comprising:
generating the recording magnetic field from the magnetic field-generating source in an oblique direction with respect to the surface of the information-recording medium so that the recording magnetic field has an in-plane component which is parallel to the surface of the information-recording medium in the irradiated area, and the in-plane component is in the same direction as a direction of a track existing in the irradiated area.
In the recording method of the present invention, the information is recorded by radiating the recording light beam onto the surface of the information-recording medium, and generating the recording magnetic field from the magnetic field-generating source in the oblique direction with respect to the surface of the information-recording medium. In this process, the recording magnetic field is generated from the magnetic field-generating source so that the recording magnetic field has the in-plane component which is parallel to the surface of the medium in the area irradiated with the recording light beam, and the in-plane component is directed in the same direction as the direction of the track existing in the irradiated area. According to the recording method as described above, it is possible to invert the magnetization of the magnetic domain in the recording layer more easier as compared with the conventional technique in which the magnetic field is applied in the perpendicular direction. Therefore, it is possible to widen the power margin for the recording light beam and the recording magnetic field. The reason of this effect will be explained below.
According to the study performed by the present inventors, it has been revealed that the recording magnetic domain can be easily formed when the component in the in-plane direction of the recording medium (hereinafter referred to as xe2x80x9cin-plane component of the recording magnetic fieldxe2x80x9d) is included as a vector component of the recording magnetic field to be applied to the information-recording medium, because of the following reason. That is, the inversion of magnetization in the recording layer is triggered by the in-plane component of the recording magnetic field. In other words, when the minute magnetic domain, which exists in the recording light beam spot in the recording layer, is heated by being irradiated with the recording light beam, the minute magnetic domain is gradually directed to the in-plane component by the aid of the in-plane component of the recording magnetic field. When the component in the perpendicular direction of the recording magnetic field is applied in the recording direction (explanation will be made below assuming that the upward direction is the recording direction) to the minute magnetic domain directed in the in-plane direction, the minute magnetic domain is inclined upwardly from the in-plane direction. This minute magnetic domain serves as a trigger, and the magnetization in a predetermined temperature area (area in which the coercive force is lowered) within the recording light beam spot in the recording layer is inverted in a chained manner to form the recording magnetic domain. The minute magnetic domain is referred to as xe2x80x9cseed magnetic domainxe2x80x9d. During the recording, the inversion of magnetization gradually occurs starting from the seed magnetic domain which is strongly affected by the in-plane magnetic field. The generation of the seed magnetic domain is called xe2x80x9cnucleationxe2x80x9d. The position, at which the seed magnetic domain is generated in the recording layer, i.e., the position, at which the inversion of magnetization firstly occurs, is called xe2x80x9cnucleation pointxe2x80x9d. In the present invention, the recording magnetic field is applied obliquely to the surface of the information-recording medium so that the magnetic field component in the in-plane direction is included. Therefore, the inversion of magnetization of the recording magnetic domain is easily caused. Further, the direction of magnetic field generation of the recording magnetic field is adjusted so that the in-plane direction of the recording magnetic field is parallel to the tangential direction of the information-recording medium when the recording magnetic field is applied to the disk-shaped medium. Therefore, it is possible to concentrate the nucleation on one place. Accordingly, the formation of the recording mark (recording magnetic domain) can be advanced from one nucleation easily and reliably. In order that the in-plane direction of the recording magnetic field is parallel to the tangential direction of the information-recording medium, for example, the magnetic coil, which is used to generate the recording magnetic field, may be arranged so that the direction of the geometrically projected component of the axis of magnetic field generation on the medium surface is parallel to the tangential direction of the medium. According to the recording medium as described above, the power margins of the recording light beam and the recording magnetic field are widened as compared with the conventional technique.
On the other hand, if the recording magnetic field is applied only in the perpendicular direction with respect to the medium as performed in the conventional technique, it is difficult to locally cause the nucleation. In this case, it is considered that the invention of magnetization is caused all at once in a unit of the size of the recording magnetic field when the magnitude of the recording magnetic field exceeds a predetermined threshold value. On the contrary, in the present invention, the in-plane component of the recording magnetic field serves to locally generate the minute seed magnetic domain the recording layer. The magnetic domain is inverted in a chained manner taking advantage of this opportunity, and thus the recording magnetic domain is formed. Accordingly, the recording magnetic domain can be formed controllably, and it is possible to easily form the minute magnetic domain. Therefore, according to the recording method of the present invention, it is possible to form the super minute magnetic domain in the controlled fashion. Thus, it is possible to expect the super high density recording.
Japanese Patent Application Laid-Open No. 7-85526 discloses a magneto-optical recording apparatus comprising a magnetic field-generating unit for applying an external magnetic field in an inclined direction with respect to a film surface of a magnetic film of a magneto-optical recording medium. However, this patent document neither describes nor suggests the fact that the magnetic field is obliquely applied so that the direction of magnetic field generation is parallel to the tangential direction of the disk-shaped medium (a direction of a track in an area o which a recording light is irradiated) to concentrate the nucleation on one piece.
In the recording method of the present invention, when the recording layer of the information-recording medium is a perpendicularly magnetizable film, it is preferable that the information-recording medium comprises a recording auxiliary layer having perpendicular magnetic anisotropy which is smaller than perpendicular magnetic anisotropy of the recording layer. It is preferable that the recording auxiliary layer is formed in contact with one surface of the recording layer. The layers may be stacked in an order of the recording layer and the recording auxiliary layer disposed adjacently to the substrate, or in an order of the recording auxiliary layer and the recording layer disposed adjacently to the substrate. When the recording auxiliary layer has perpendicular magnetization upon the recording, i.e., when the recording auxiliary layer has perpendicular magnetization within a recording temperature range (200xc2x0 C. to 350xc2x0 C.), then the magnetization of the recording auxiliary layer is aligned to the recording magnetic field during the recording to cause the inversion of magnetization in accordance with the nucleation prior to the magnetic domain in the recording layer. As a result, the inversion of magnetization of the recording magnetic domain in the recording layer tends to occur by the aid of the trigger of the inversion of magnetization in the recording auxiliary layer.
On the other hand, when the recording auxiliary layer has in-plane magnetization upon the recording, i.e., when the recording auxiliary layer has in-plane magnetization within the recording temperature range, then the in-plane magnetization, which is directed in an arbitrary direction in the recording auxiliary layer, is aligned in the direction of the in-plane component of the recording magnetic field during the recording. Therefore, the inversion of magnetization tends to occur in the recording layer by the aid of the trigger of the in-plane magnetization aligned in the certain direction in the recording auxiliary layer.
In the present invention, those usable as a material for constructing the recording auxiliary layer include, for example, GdFeCo, GdFe, GdCo, GdDyFe, GdDyCo, Gd-based alloy added with transition metal, transition metal-noble metal alloy, and multilayered film composed of the above. It is also possible to use, for example, alloy composed of CoCr alloy and containing high melting point metal such as Ta, Pt, and Mo, and granular alloy composed of Co-Pd alloy or Co-Pt alloy and non-magnetic substance.
In the present invention, it is preferable to use a perpendicularly magnetizable film which exhibits perpendicular magnetic anisotropy at a temperature of not less than the room temperature, as a material for constructing the recording layer of the information-recording medium. For example, it is most preferable to use amorphous alloy composed of transition metal and rare earth such as TbFeCo, DyFeCo, and TbDyFeCo. Further, for example, it is also possible to stack, on the recording layer, an alternate stacked member of Pt film and Co film and garnet-based oxide magnetic member. Those preferably usable also include CoCr-based alloy film composed of perpendicular magnetic recording material used for hard disk, added with, for example, CoPt-based alloy film having in-plane magnetization component, and Pt/Co perpendicularly magnetizable film added with CoCr-based alloy film having in-plane magnetization component.
In the recording method of the present invention, in order to apply the recording magnetic field in the oblique direction with respect to the surface of the information-recording medium, for example, a magnetic coil for generating the recording magnetic field as having been hitherto used may be actively inclined to apply the recording magnetic field so that the direction of the magnetic field generated from the magnetic coil (axis of magnetic field generation) is oblique with respect to the surface of the information-recording medium.
According to a second aspect of the present invention, there is provided a recording method on an information-recording medium for recording information on the information-recording medium including a recording layer by radiating a recording light beam while applying a recording magnetic field, wherein the information-recording medium includes a recording auxiliary layer in contact with the recording layer, the recording auxiliary layer having perpendicular magnetic anisotropy smaller than perpendicular magnetic anisotropy of the recording layer, the method comprising:
applying the recording magnetic field in an oblique direction to a surface of the information-recording medium.
According to the recording method of the present invention, the recording magnetic field is applied in the oblique direction to the information-recording medium including the recording auxiliary layer in contact with the recording layer. The recording auxiliary layer is constructed by using a magnetic material having the perpendicular magnetic anisotropy which is smaller than that of the recording layer. Accordingly, when the recording magnetic field is applied in the oblique direction, the magnetization in the recording auxiliary layer is inverted prior to the magnetization in the recording layer. Taking advantage of the opportunity of the inversion of magnetization in the recording auxiliary layer, the magnetization in the recording layer is easily inverted by the aid of the exchange coupling force. Thus, the information can be recorded with ease as compared with the conventional technique.
According to a third aspect of the present invention, there is provided a recording method on an information-recording medium for recording information on the information-recording medium including a recording layer by radiating a recording light beam while applying a recording magnetic field, the method comprising:
applying the recording magnetic field to a surface of the information-recording medium not only in a perpendicular direction but also in an in-plane direction.
According to the recording method as described above, the intensity and the application timing of the recording magnetic field in the in-plane direction may be made different from those of the recording magnetic field in the perpendicular direction. Further, the application direction of the in-plane recording magnetic field may be made different as well between the process in which information is recorded and the process in which information is erased.
According to a fourth aspect of the present invention, there is provided a recording apparatus for an information-recording medium including a perpendicular magnetic recording layer, comprising:
a recording magnetic field-generating unit having an axis of magnetic field generation, for generating a recording magnetic field along the axis to the disk-shaped information-recording medium; and
a light source for radiating a recording light beam onto the information-recording medium, wherein:
the recording magnetic field-generating unit is arranged so that the axis of magnetic field generation is directed in the oblique direction with respect to the direction of magnetization of the recording layer of the information-recording medium, and a geometrically projected component of the axis of magnetic field generation on a medium surface is directed in a direction of a track existing in an area irradiated with the recording light beam.
The recording magnetic field-generating unit may be also arranged such that the axis of magnetic field generation is parallel to the optical axis of the recording light beam, and it is deviated from the optical axis of the recording light beam in the in-plane direction of the information-recording medium. Alternatively, the recording magnetic field-generating unit may be arranged such that the axis of magnetic field generation of the recording magnetic field-generating unit obliquely intersects the optical axis of the recording light beam in the information-recording medium.
According to a fifth aspect of the present invention, there is provided a recording apparatus for an information-recording medium, comprising:
a recording magnetic field-generating unit for applying a recording magnetic field to the information-recording medium including a perpendicular magnetic recording layer; and
a light source for radiating a recording light beam onto the information-recording medium, the recording magnetic field-generating unit including:
a first magnetic field-generating unit for generating a magnetic field in a perpendicular direction with respect to a surface of the information-recording medium; and
a second magnetic field-generating unit for generating a magnetic field in an in-plane direction with respect to the surface of the information-recording medium.
The recording apparatus of the present invention includes the second magnetic field-generating unit with which the magnetic field can be generated in the in-plane direction. Therefore, it is easy to cause the inversion of magnetization of the recording magnetic domain of the information-recording medium. The intensity and the application timing of the magnetic field generated by the second magnetic field-generating unit, and the alternating cycle when an alternating magnetic field is generated are not necessarily the same as those of the first magnetic field-generating unit, and it is possible to make adjustment so that they are different from each other. When the information-recording medium has a dark-shaped configuration, it is preferable that the direction of the axis of magnetic field generation of the second magnetic field-generating unit is set to be parallel to the direction of a track existing in an area irradiated with the recording light beam.
The recording apparatus of the present invention may comprise a lens for collecting and radiating the recording light beam onto the information-recording medium. The lens may be a high NA lens having an NA (numerical aperture) of not less than 0.5, preferably not less than 0.6. More preferably, the recording apparatus of the present invention comprises a solid immersion lens. The use of the solid immersion lens effectively increases the numerical aperture of the lens. Therefore, it is possible to decrease the light spot diameter. Accordingly, information can be recorded at a super high density. When the high NA lens and/or the solid immersion lens is used, it is preferable that at least one of the first and second magnetic field-generating units is installed under or around the high NA lens or the solid immersion lens.
According to a sixth aspect of the present invention, there is provided an information-recording medium comprising a recording layer and a reproducing layer, the medium further comprising:
a reproducing auxiliary layer having perpendicular magnetic anisotropy smaller than perpendicular magnetic anisotropy of the recording layer, the reproducing auxiliary layer being disposed between the recording layer and the reproducing layer.
In the present invention, the reproducing auxiliary layer, which has the perpendicular magnetic anisotropy smaller than that of the recording layer, is formed between the recording layer and the reproducing layer. Accordingly, the recording magnetic domain in the recording layer can be easily transferred to the reproducing layer. Therefore, it is possible to widen the power margins of the reproducing light beam and the reproducing magnetic field. The reason of this effect will be explained below.
According to the study performed by the present inventors, the following fact has been revealed concerning the information-recording medium in which information is reproduced by transferring the recording magnetic domain in the recording layer to the reproducing layer. That is, the inversion of magnetization in the reproducing layer is caused by the trigger of the in-plane component of the leak magnetic field from the recording magnetic domain. As shown in FIG. 8A, the leak magnetic field from a recording magnetic domain 201 in a recording layer 600 includes a perpendicular component 210 which outgoes in the perpendicular direction, as well as in-plane components, i.e., in-plane components 211, 212 directed toward non-recording magnetic domains 202, 203 (magnetization in the downward direction in the drawing) located adjacent to the recording magnetic domain respectively. It is considered that minute magnetic domains 221, 222 of a reproducing layer 300, which are located substantially just over boundaries between the recording magnetic domain 201 and the non-recording magnetic domains 202, 203, are gradually directed toward the in-plane direction by the aid of the in-plane components 211, 212 of the leak magnetic field, when they are heated by being irradiated with the reproducing light beam respectively. When a reproducing magnetic field Hr is applied to the upward direction to the minute magnetic domains 221, 221 having been directed in the in-plane direction, the minute magnetic domains 221, 222 are inclined to the upward direction from the in-plane direction respectively (FIG. 8B). The magnetic domains 221, 222 serve as the trigger (taking advantage of the opportunity) so that all of magnetic domains 213 in a predetermined area in the reproducing light beam spot in the reproducing layer make inversion in a chained manner (FIG. 8C). The minute magnetic domains 221, 222 are seed magnetic domains. During the reproduction, the inversion of magnetization is gradually caused from the seed magnetic domains 221, 222 disposed substantially just over the boundaries between the recording magnetic domain 201 and the non-recording magnetic domains 202, 203. Therefore, it is considered that if the magnetic field in the in-plane direction can be increased at the positions of occurrence of the inversion of magnetization, i.e., at the nucleation points substantially just over the boundaries between the recording magnetic domain 201 and the non-recording magnetic domains 202, 203, then it is easy to cause the inversion of magnetization in the reproducing layer. In the present invention, the reproducing auxiliary layer, which exists between the recording layer and the reproducing layer, is composed of a material having perpendicular magnetic anisotropy smaller than that of the recording layer. Therefore, the magnetization of the reproducing auxiliary layer tends to be aligned in the direction of the leak magnetic field of the recording magnetic domain. Thus, the leak magnetic field in the in-plane direction from the recording magnetic domain is remarkably emphasized. Especially, when the saturation magnetization of the reproducing auxiliary layer is made larger than the saturation magnetization of the recording layer, it is possible to further emphasize the leak magnetic field in the in-plane direction. Therefore, the inversion of magnetization of the seed magnetic domain in the reproducing layer is easily caused by the amplified in-plane component from the reproducing auxiliary layer. Accordingly, the power margins of the reproducing light beam and the reproducing magnetic field are widened as compared with the conventional technique.
In order to reproduce recorded information on the information-recording medium of the present invention, it is desirable that the reproducing light beam is radiated, and the reproducing magnetic field is applied obliquely with respect to the medium. When the reproducing magnetic field is applied obliquely to the information-recording medium as shown in FIG. 9, then the magnetic field 311 in the in-plane direction at the nucleation point P1 is increased by the in-plane component Hx of the reproducing magnetic field, and the magnetic field 312 in the in-plane direction at the nucleation point P2 is decreased. Therefore, it is possible to cause the inversion of magnetization of the magnetic domain in the reproducing layer at the nucleation point P1 more easily. With reference to FIG. 9, P3 exists as the nucleation point at which the magnetic field in the in-plane direction is increased. The nucleation point P3 exists within the reproducing spot when the non-recording magnetic domain 303 is subjected to reproduction. Therefore, the inversion of magnetization may occur in the reproducing layer by the trigger of the nucleation point P3. However, this phenomenon can be avoided by deviating the phase of the alternating magnetic field, i.e., by deviating the phase of the magnetic field (reproducing magnetic field) +Hr in the direction to cause the inversion of magnetization in the reproducing layer so that the application timing of the reproducing magnetic field is changed. The reproducing magnetic field is not necessarily applied obliquely with respect to the information-recording medium. Alternatively, it is also preferable that two reproducing magnetic fields may be used to apply the magnetic fields in the x direction and in the y direction respectively.
In the present invention, those usable as a material for constructing the reproducing auxiliary layer include, for example, GdFeCo, GdFe, GdCo, GdDyFe, GdDyCo, Gd-based alloy added with transition metal, transition metal-noble metal alloy, and multilayered film composed of the above.
The information-recording medium of the present invention may be constructed such that the reproducing auxiliary layer contacts with the recording layer. It is preferable that a non-magnetic layer is allowed to intervene between the reproducing auxiliary layer and the reproducing layer. Alternatively, it is also possible that a magnetic layer is allowed to intervene between the reproducing auxiliary layer and the reproducing layer, provided that the magnetic layer is non-magnetic when the recording magnetic domain in the recording layer is transferred to the reproducing layer. The information-recording medium having such a structure is preferably used as a magneto-optical recording medium for MAMMOS. In this case, the reproducing layer can function as a MAMMOS layer (magnetic domain-magnifying reproducing layer).
The information-recording medium of the present invention may be constructed such that the reproducing auxiliary layer contacts with the reproducing layer. In this arrangement, it is preferable that a non-magnetic layer or a magnetic layer which is non-magnetic when the recording magnetic domain in the recording layer is transferred to the reproducing layer is allowed to intervene between the reproducing auxiliary layer and the recording layer. The information-recording medium having such a structure is preferably used as a magneto-optical recording medium for MSR, which is applicable to the FAD (Front Aperture Detection) system, the RAD (Rear Aperture Detection) system, and the CAD (Center Aperture Detection) system.
In the present invention, it is preferable that the reproducing auxiliary layer has a temperature gradient of saturation magnetization of not less than 5 emu/cm3 per 10xc2x0 C. within a temperature range of 160xc2x0 C. to 200xc2x0 C. More preferably, the reproducing auxiliary layer has a temperature gradient of saturation magnetization of not less than 8 emu/cm3 per 10xc2x0 C. within a temperature range of 100xc2x0 C. to 160xc2x0 C. In order to satisfy such a condition, for example, adjustment may be made such that the compensation temperature of the material for constructing the reproducing auxiliary layer is higher than the compensation temperature of the material for constructing the recording layer. In the present invention, the temperature gradient of saturation magnetization means an average temperature gradient of saturation magnetization within a specified temperature range.
According to a seventh aspect of the present invention, there is provided an information-recording medium comprising a recording layer and a reproducing layer, wherein:
the recording layer has a temperature gradient of saturation magnetization of not less than 5 emu/cm3 per 10xc2x0 C. within a temperature range of 160xc2x0 C. to 200xc2x0 C.
In the present invention, the recording layer is constructed by using a material having, in average, a temperature gradient of saturation magnetization of not less than 5 emu/cm3 per 10xc2x0 C. within a temperature range of 160xc2x0 C. to 200xc2x0 C. More preferably, the recording layer has, in average, a temperature gradient of saturation magnetization of not less than 8 emu/cm3 per 10xc2x0 C. within a temperature range of 100xc2x0 C. to 160xc2x0 C. Accordingly, as understood from a distribution D1 of the component in the perpendicular direction of the lack magnetic field shown in FIG. 13, the leak magnetic field in the perpendicular direction from the recording magnetic domain M is suddenly increased from the area outside of the reproducing light beam spot S toward the center. That is, as understood from comparison with a conventional distribution D2 of the perpendicular component of the leak magnetic field, the contrast of the leak magnetic field, which depends on the temperature distribution in the recording layer in the reproducing light beam spot S, is distinct as compared with the conventional one. Therefore, the individual minute magnetic domains in the recording layer are reliably transferred to the reproducing layer. Further, the magnetic domains are easily magnified when the reproducing layer is a magnetic domain-magnifying reproducing layer. On the other hand, when the temperature gradient of saturation magnetization of the recording layer is adjusted, the leak magnetic field in the in-plane direction, which is generated from the recording magnetic domain, is decreased. Therefore, even when any non-recorded magnetic domain exists in the recording layer, the magnetic domain in the reproducing layer, which is located just over the non-recorded magnetic domain, is prevented from inversion which would be otherwise caused by the reproducing magnetic field. The occurrence of nucleation may be also facilitated on the information-recording medium according to the seventh aspect of the present invention by applying the reproducing magnetic field in the in-plane direction or in the oblique direction so that the in-plane component of the magnetic field is added. Those usable as a material for the recording layer which satisfies the foregoing condition include, for example, heavy rare earth-transition metal alloy such as GdTbFeCo, TbFeCo, and TbDyFeCo.
Further, the recording layer may have a two-layered structure composed of a first recording layer and a second recording layer. In this arrangement, the combined magnetization component of the saturation magnetization of the first recording layer and the saturation magnetization of the second recording layer has, in average, a temperature gradient of 5 emu/cm3 per 10xc2x0 C. within a temperature range of 160xc2x0 C. to 200xc2x0 C. For example, when a conventional recording layer material (TbFeCo), which has a temperature dependency of saturation magnetization as shown in FIG. 10, is used for the second recording layer in which the recording magnetic domain is formed, it is preferable that the first recording layer is constructed by using a material, for example, GdFeCo which has a temperature gradient steeper than the temperature gradient of the second recording layer. FIG. 11 shows a temperature dependency of saturation magnetization of a single layer of TbFeCo (second recording layer), a temperature dependency of saturation magnetization of a single layer of GdFeCo (first recording layer) having a film thickness which is a half of that of the TbFeCo layer, and a temperature dependency of saturation magnetization of a combined recording film constructed by them. As understood from the graph shown in FIG. 11, when the two-layered structure is formed by providing the material (GdFeCo) having the steep temperature gradient of saturation magnetization together with the conventional recording layer (TbFeCo) having the gentle temperature gradient of saturation magnetization, it is possible to increase the temperature gradient of saturation magnetization (combined saturation magnetization) in the reproducing temperature area. The value of the saturation magnetization of the combined recording layer in the graph shown in FIG. 11 is represented by an average value determined by being weighted with the film thickness ratio of the TbFeCo layer and the GdFeCo layer.
According to an eighth aspect of the present invention, there is provided a reproducing method on an information-recording medium for reproducing magnetization information of a magnetic domain transferred from a recording layer to a reproducing layer by radiating a reproducing light beam while applying a reproducing magnetic field to the information-recording medium including the recording layer and the reproducing layer, the method comprising:
applying the reproducing magnetic field in an oblique direction to a surface of the information-recording medium.
In the reproducing method of the present invention, in order to apply the reproducing magnetic field in the oblique direction with respect to the surface of the information-recording medium, for example, a magnetic coil for generating the reproducing magnetic field as having been hitherto used may be actively inclined and arranged to apply the reproducing magnetic field so that the direction of the magnetic field generated from the magnetic coil is oblique with respect to the surface of the information-recording medium.
According to a ninth aspect of the present invention, there is provided a reproducing method on an information-recording medium for reproducing magnetization information of a magnetic domain transferred from a recording layer to a reproducing layer by radiating a reproducing light beam while applying a reproducing magnetic field to the information-recording medium including the recording layer and the reproducing layer, the method comprising:
applying the reproducing magnetic field to a surface of the information-recording medium not only in a perpendicular direction but also in an in-plane direction.
In the reproducing method as described above, at least one reproducing magnetic field of the reproducing magnetic field applied in the perpendicular direction and the reproducing magnetic field applied in the in-plane direction may be an alternating magnetic field. When both of the reproducing magnetic field applied in the perpendicular direction and the reproducing magnetic field applied in the in-plane direction are alternating magnetic fields, it is also possible to make adjustment so that at least one of the magnetic field intensity, the magnetic field application timing, and the alternating cycle of the alternating magnetic fields differs. For example, as shown in FIG. 14, the reproducing alternating magnetic field in the in-plane direction is applied to make synchronization with the recording clock used when the recording mark is formed, and the reproducing alternating magnetic field in the perpendicular direction is applied at a cycle which is a half of that of the recording clock. In this process, the reproducing alternating magnetic field is altered so that the in-plane component of the leak magnetic field is increased at each of the leading edge and the trailing edge of the recording mark. When the leading edge and the trailing edge of the recording mark are detected, then the cycle of the reproducing alternating magnetic field in the in-plane direction is synchronized with the recording clock, and thus the in-plane component of the reproducing magnetic field applied thereto can be made mutually different. When the cycle of the reproducing alternating magnetic field in the perpendicular direction is a half of the cycle of the recording clock, the magnetic fields in the recording direction and in the erasing direction, which are perpendicular to the film surface, are applied to each of the leading edge and the trailing edge of the recording mark. Accordingly, the nucleation occurs at the leading edge and the trailing edge of the recording mark, and reproduced signals can be detected from the leading edge and the trailing edge of one recording mark respectively.
According to a tenth aspect of the present invention, there is provided a reproducing apparatus for an information-recording medium, comprising a reproducing magnetic field-generating unit for applying a reproducing magnetic field to the information-recording medium including a perpendicular magnetic recording layer and a reproducing layer, wherein:
the reproducing magnetic field-generating unit is arranged so that the magnetic field is generated in an oblique direction with respect to a magnetization direction of the perpendicular magnetic recording layer.
In the reproducing apparatus of the present invention, it is preferable that the reproducing magnetic field-generating unit has its axis of magnetic field generation which is arranged so that the magnetic field is generated in the oblique direction with respect to the direction of magnetization of the recording layer of the information-recording medium.
Alternatively, the reproducing magnetic field-generating unit may be arranged such that its axis of magnetic field generation is parallel to the optical axis of the reproducing light beam, and the axis of magnetic field generation is disposed at a position deviated from the optical axis of the reproducing light beam in the in-plane direction of the information-recording medium. Alternatively, the reproducing magnetic field-generating unit may be arranged such that the axis of magnetic field generation of the reproducing magnetic field-generating unit obliquely intersects the optical axis of the reproducing light beam in the information-recording medium.
According to an eleventh aspect of the present invention, there is provided a reproducing apparatus for an information-recording medium, comprising a reproducing magnetic field-generating unit for applying a reproducing magnetic field to the information-recording medium including a perpendicular magnetic recording layer and a reproducing layer, the reproducing magnetic field-generating unit including:
a first magnetic field-generating unit for generating a magnetic field in a perpendicular direction with respect to a surface of the information-recording medium; and
a second magnetic field-generating unit for generating a magnetic field in an in-plane direction with respect to the surface of the information-recording medium.
The reproducing apparatus of the present invention includes the second magnetic field-generating unit which makes it possible to generate the magnetic field in the in-plane direction. Therefore, it is easy to cause the inversion of magnetization in the reproducing layer of the information-recording medium. The second magnetic field-generating unit may be constructed, for example, by using a permanent magnet or a single magnetic pole type magnetic field-generating unit (electromagnet) comprising a magnetic coil wound around a columnar magnetic core. When the second magnetic field-generating unit is constructed by using the single magnetic pole type magnetic field-generating unit, the reproducing magnetic field in the in-plane direction, which is generated from the magnetic field-generating unit, may be either an alternating magnetic field or a direct current magnetic field. The magnetic field intensity and the application timing of the reproducing magnetic field generated from the second magnetic field-generating unit, and the alternating cycle in the case of the alternating magnetic field may be appropriately adjusted so that they are different from those of the first magnetic field-generating unit.
The reproducing apparatus of the present invention may include a lens for collecting and radiating the reproducing light beam onto the information-recording medium. The lens may be a high NA lens having an NA (numerical aperture) of not less than 0.5, preferably not less than 0.6. More preferably, the reproducing apparatus of the present invention includes a solid immersion lens. The use of the solid immersion lens effectively increases the numerical aperture of the lens. Therefore, it is possible to decrease the light spot diameter. Accordingly, information recorded at a super high density can be reproduced. When the high NA lens and/or the solid immersion lens is used, it is preferable that at least one of the first and second magnetic field-generating units is installed under or around the high NA lens or the solid immersion lens.